Correctly it’s a smoke detector, we have to put it in every bedroom when we built a new house. Fortunately it was malfunction, maybe because of the exhaust from the air conditioner and the futon dryer. But you might imagine, its sound was really alarming, rather threatening!
We have lived in this small house for more than 6 years now and this is the first time we hear this alarm. I really hope I would never hear it again!! X(